Preparation of maleic anhydride

ABSTRACT

The present invention provides a process for the preparation of oxidation catalysts containing mixed oxides of vanadium and phosphorus, which catalysts are particularly effective in the oxidation of n-butane, n-butenes, 1,3-butadiene or a mixture thereof with molecular oxygen or an oxygen-containing gas in the vapor phase to produce high yields of maleic anhydride with good selectivity. A vanadium compound is introduced into an olefinic oxygenated organic liquid medium, a phosphorus-containing compound is added to the medium, reduction of at least a portion of the vanadium to a +4 valence state is effected either prior to or subsequent to the addition of the phosphorus-containing compound, and the resulting vanadium-phosphorus oxide catalyst precursor is recovered, dried and calcined.

BACKGROUND OF THE INVENTION

This invention relates to a process for preparing catalysts useful inthe production of dicarboxylic acid anhydrides by the oxidation ofhydrocarbons. More particularly it is directed to the preparation ofcatalysts suitable for producing maleic anhydride from 4-carbon atomhydrocarbons, such as n-butane, n-butenes, 1,3 butadiene or a mixturethereof.

Catalysts containing vanadium and phosphorus oxides have been used inthe oxidation of 4-carbon atom hydrocarbons, such as n-butane,n-butenes, 1,3 butadiene or mixtures thereof with molecular oxygen oroxygen-containing gas to produce maleic anhydride. Conventional methodsof preparing these catalysts involve combining a vanadium compound, aphosphorus compound, and if desired, promoter element compounds in areducing medium under conditions which will provide vanadium in avalence state below +5 to form catalyst precursors capable of beingconverted to an oxide. The catalyst oxide precursor is then recoveredand calcined to provide active catalytic material.

The use of gaseous HCl as a reducing agent for vanadium is disclosed inU.S. Pat. No. 4,002,650 where the vanadium and phosphorus components arepresent in an aqueous solution. The use of gaseous HCl as a reducingagent for vanadium is also described in U.S. Pat. No. 4,043,943 wherethe vanadium and phosphorus components are reacted in a liquid saturatedorganic medium.

Similar preparational techniques are described in European patentapplication No. 3,431 in which the additional step of comminuting thevanadium-phosphorus precursor to a particle size of 500 to 700 microns(0.5 to 0.7 mm) is disclosed.

The use of such reducing agents as disclosed in the art, requiresspecial precautions in the preparation of these catalysts because of thecorrosive nature of the materials utilized.

U.S. Pat. No. 4,016,105 describes the preparation of vanadium andphosphorus oxide-containing catalyst, utilizing as reducing agentsorganic acids including oxalic, citric, formic, ascorbic and malic, oraldehydes, including formaldehyde and acetaldehyde, together with aco-reducing secondary alcohol. These reducing agents are added to anaqueous solution containing the vanadium and phosphorus components.

A method for preparing catalysts containing vanadium and phosphorusoxides was described in U.S. Pat. Nos. 4,132,670 and 4,187,235 includingforming a vanadium-containing compound dispersion in an organic liquidmedium such as alcohols, aldehydes, ketones, ethers or mixtures thereof,heating the dispersion to reduce the vanadium, and adding phosphoricacid in an organic solvent. These patents teach that organic compoundssatisfactory for use in the preparation of such oxides must not containolefin double bonds.

In the prior art, separation and recovery of the catalyst precursor fromthe reaction solution has provided difficulties where HCl gas is used asa reducing agent for vanadium, containment and disposal of the excess,corrosive gas required constitutes a problem in commercial scale-up ofthe catalyst preparation. Conventionally, the solution containing theprecursor must be evaporated down, usually to a catalystprecursor-containing paste which must then be dried, broken up andground. This too provides difficulties for the commercial scale-up ofthe process, particularly where the catalyst precursor-containingsolution includes flammable organic liquids.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a process forpreparing vanadium and phosphorus-containing oxidation catalysts.

It is a further object of the invention to provide a process ofpreparing vanadium and phosphorus-containing catalysts useful for theoxidation of 4-carbon atom hydrocarbons to produce maleic anhydride,which catalysts exhibit high yields and selectivity to maleic anhydride.

It is a further object of the invention to provide a process ofpreparing vanadium and phosphorus-containing catalysts useful for theoxidation of 4-carbon atom hydrocarbons to produce maleic anhydridewhich utilizes mild reducing agents to effect an average valence stateof about +3.5 to about +4.6 in the vanadium present in the catalyst.

It is a further object of the invention to provide a process ofpreparing vanadium and phosphorus-containing catalysts useful for theoxidation of 4-carbon atom hydrocarbons to produce maleic anhydridewhich is simplified, economical and avoids the hazards of corrosionand/or flammibility, and is capable of commercial scale-up.

It is a further object of the invention to provide a process ofpreparing vanadium and phosphorus-containing catalysts useful for theoxidation of 4-carbon atom hydrocarbons to produce maleic anhydridewhich includes improved recovery of catalyst precursors from thereaction medium.

These and other objects, together with the advantages thereof over knownmethods, which shall be apparent from the specification which follows,are accomplished by the invention as hereinafter described and claimed.

In general the process of the present invention comprises the steps of

(a) introducing a pentavalent vanadium-containing compound into anolefinic, oxygenated organic liquid-containing liquid medium;

(b) effecting reduction of at least a portion of said vanadium to avalence state of +b 4;

(c) adding a phosphorus-containing compound to said medium to form acatalyst precursor precipitate;

(d) recovering the catalyst precursor precipitate;

(e) drying the catalyst precursor precipitate;

(f) calcining the catalyst precursor precipitate.

Another embodiment of the process of the present invention comprises thesteps of

(a) introducing a pentavalent vanadium-containing compound into anolefinic oxygenated organic liquid-containing medium capable of at leastpartially solubilizing the vanadium;

(b) effecting reduction of at least a portion of said vanadium to avalence state of about +4;

(c) removing unsolubilized vanadium-containing compounds having aparticle size greater than about 0.1 mm diameter;

(d) adding a phosphorus-containing compound to the reaction mediumresulting from step (c) to form a catalyst precursor precipitate;

(e) recovering the catalyst precursor precipitate;

(f) drying the catalyst precursor precipitate; and

(g) calcining the catalyst precursor precipitate.

Yet another embodiment of the process of the present invention comprisesthe steps of

(a) introducing a pentavalent vanadium compound and a phosphoruscompound into an olefinic oxygenated organic liquid-containing medium;

(b) effecting reduction of at least a portion of the vanadium to avalence state of about +4 while in the presence of the phosphoruscompound to form a vanadium-phosphorus mixed oxide precursor;

(c) recovering the vanadium phosphorus mixed oxide catalyst precursor;

(d) drying said catalyst precursor; and

(e) calcining the catalyst precursor.

The catalysts prepared by the above process are particularly effectivein the oxidation of 4-carbon atom hydrocarbons such as n-butane,n-butenes, 1,3 butadiene or mixtures thereof with molecular oxygen or anoxygen-containing gas in the vapor phase to produce high yields ofmaleic anhydride with high selectivity. Essentially all the productproduced in this oxidation process is maleic anhydride, with only minoramounts of lower acids being detected.

DETAILED DESCRIPTION OF THE INVENTION

In the process for the preparation of an oxidation catalyst containingthe mixed oxides of vanadium and phosphorus, a vanadium compound,particularly a pentavalent vanadium compound, is introduced into anolefinic, oxygenated organic liquid medium. Suitable vanadium compoundscontaining pentavalent vanadium include: vanadium pentoxide or vanadiumsalts, such as ammonium metavanadate and vanadium oxytrihalides.Vanadium pentoxide is preferred.

In one embodiment of the invention, the pentavalent vanadium containingcompound is at least partially solubilized in said olefinic oxygenatedorganic liquid medium. To aid in solubilizing the vanadium, it ispreferred that the vanadium-containing compound which is introduced intothe liquid medium have a small particle size, and methods for furtherreducing particle size of the vanadium compound while in the liquidmedium, such as by ball milling the initial suspension of vanadium inthe liquid medium, may be employed.

We have discovered that olefinic, oxygenated organic liquid, contrary tothe teachings of the prior art, are suitable, mild reducing agents forvanadium, capable of reducing pentavalent vanadium to an average valenceof between +3.5 and +4.6, that is they are capable of reducing at leasta portion of the pentavalent vanadium introduced into such a liquid to a+4 valence state. Such partial reduction of vanadium permits its use inthe preparation of catalysts useful in the production of maleicanhydride from 4-carbon atom hydrocarbons. A harsher reducing agentwould reduce the vanadium to a much lower valence state, possibly tobase metal, in both cases, rendering the vanadium virtually inactive,catalytically.

As used in this specification, the term "olefinic oxygenated organicliquids" refers to compounds containing at least the elements carbon,hydrogen and oxygen, said compounds having at least one non-aromaticcarbon to carbon double bond and may include compounds having otherfunctional groups provided that these functional groups do not adverselyeffect the formation of recoverable mixed oxides of vanadium andphosphorus, wherein said vanadium has an average valence of about +3.5to about +4.6. These compounds must be liquid at some temperature withinthe range of temperatures between about 20° C. to about 120° C.

The olefinic, oxygenated organic liquids utilized act as solvents forphosphoric acid and are relatively unreactive towards phosphoric acid.These liquids are not, however, solvents for the mixed oxide of vanadiumand phosphorus. Suitable olefinic oxygenated organic liquids for use inthe invention include compounds such as olefinic alcohols, olefinicaldehydes, olefinic ketones, olefinic acids, olefinic ethers, olefinicepoxides and mixtures of the above. Preferred organic liquids suitablefor use in this invention are olefinic alcohols, particularly allylalcohol, methallyl alcohol and crotyl alcohol.

The liquid medium used in the process of the present invention may becomprised of substantially all olefinic oxygenated organic liquids, orit may additionally include other organic reducing organic liquids forvanadium such as isobutanol, or non-reducing liquids for vanadium suchas tert butyl alcohol. The liquid medium used in the process of thepresent invention is preferably essentially anhydrous.

After the pentavalent vanadium compound is introduced into the liquidmedium, reduction of the vanadium is effected either prior to orsubsequent to the addition of a phosphorus-containing compound to theliquid medium. The reduction is effected preferably by heating theresulting reaction medium, with stirring if desired. Preferred vanadiumand phosphorus oxide catalysts for the oxidation of 4-carbon atomhydrocarbons to maleic anhydride contain vanadium in an average valencestate of about +3.5 to about +4.6. This average valence state isachieved when at least a portion of the pentavalent vanadium introducedinto the reaction mixture is reduced to the +4 state. The averagevalence state of the vanadium is reduced preferably to about +4.1.

After partial reduction of the vanadium, in one embodiment of theinvention, unsolubilized vanadium-containing compounds are removed fromthe reaction mixture. While the unsolubilized vanadium-containingcompounds generally contain some portion of vanadium in a valence stateless than +5, the greater portion of vanadium present remains in a +5valence state. Although it is preferred in this embodiment to remove allunsolubilized vanadium-containing compounds from the liquid medium aftereffecting reduction of the vanadium, removing all such unsolubilizedvanadium-containing compounds having a particle size greater than about0.1 mm diameter, results in the production of catalysts exhibitingexcellent activity for the preparation of maleic anhydride, producinghigh yields at high selectivity. In a preferred mode of this embodimentof the process of the invention, all unsolubilized vanadium-containingcompounds having a particle size greater than about 0.04 to about 0.06mm diameter are removed. Removal is achieved by conventional means, suchas filtration, centrifugation, decantation and the like. After removalof unsolubilized vanadium-containing compounds from the liquid reactionmedium, in this embodiment of the invention, a pentavalentphosphorus-containing compound is added to the reaction medium.

Suitable phosphorus compounds containing pentavalent phosphorus include:phosphoric acid, phosphorus pentoxide, or phosphorus perhalide, such asphosphorus pentachloride. Phosphoric acid and phosphorus pentoxide arepreferred. The pentavalent phosphorus-containing compound is preferablyadded to the reaction medium in the form of a solution of thephosphorus-containing compound in either a component of the liquidreaction medium, or in a liquid capable of yielding thephosphorus-containing compound to the liquid reaction medium. Afteraddition of the phosphorus-containing compound to the liquid reactionmedium, it is preferable to heat the liquid reaction medium, withstirring, if necessary.

In other embodiments of the invention, the phosphorus-containingcompound, as described above, is added to the liquid medium (containingolefinic oxygenated organic liquids) either before reduction of thepentavalent vanadium substantially occurs, or after such reduction, withno pre-reduction filtration of unsolubilized vanadium compounds. Whenreduction of the vanadium is effected in the presence of thephosphorus-containing compound, the resulting solids dispersed in theliquid medium are the vanadium-phosphorus mixed oxide precursors, to berecovered, dried and calcined.

As stated above, the liquid medium employed should not be a solvent forthe vanadium-phosphorus mixed oxide. Therefore, even in embodiments ofthe invention in which the vanadium is at least partially solubilized,as the vanadium-phosphorus oxide catalyst precursor is formed, theprecursor precipitates out of the solution. The total H₂ O content ofthe medium, particularly at this point, should be below about 5%. Thecatalyst precursor or catalyst precursor precipitate is then recoveredfrom the reaction medium by conventional methods including filtration,centrifugation and decantation.

The catalyst precursor or catalyst precursor precipitate is dried andcalcined at a temperature of 250° C. to 600° C., preferably in thepresence of an oxygen-containing gas.

It is within the scope of this invention, to include promoterelement-containing compounds in the reaction mixture at a suitablepoint, either prior to or subsequent to reduction of the vanadium, inorder that the catalyst precursor or catalyst precursor precipitatecontain the promoter element.

Catalysts prepared by this method may exhibit a phosphorus to vanadiumratio of about 0.5:1 to about 2:1. Preferred is a P/V ratio of about0.9:1 to about 1.3:1. The catalyst is activated by calcining it in airor an oxygen-containing gas at a temperature of 250° C. to 600° C. for aperiod of up to 5 hours or more. Activation of the catalyst isaccomplished by passing a mixture of steam and air or air alone over thecatalyst at a temperature of about 300° C. to 500° C. for a period ofabout 1 to 5 hours.

The hydrocarbon reacted to form maleic anhydride may be n-butane,n-butenes, 1,3-butadiene, or a mixture thereof. Preferred is the use ofn-butane or a mixture of hydrocarbons that are produced in refinerystreams. The molecular oxygen needed for the reaction to produce maleicanhydride is most conveniently added as air, but synthetic streamscontaining molecular oxygen are also suitable. In addition to thehydrocarbon and molecular oxygen, other gases may be added to thereactant feed. For example, steam or nitrogen could be added to thereactants.

The ratio of the reactants may vary widely and are not critical. Theratio of molecular oxygen to the hydrocarbon may range from about 2 toabout 30 moles of oxygen per mole of hydrocarbon. Preferredoxygen/hydrocarbon ratios are about 4 to about 20 moles of oxygen permole of hydrocarbon.

The reaction temperature may vary widely and is dependent upon theparticular hydrocarbon and catalyst employed. Normally, temperatures ofabout 250° C. to about 600° C. are employed with temperatures of 350° C.to 500° C. being preferred.

The catalyst may be used alone or a support could be employed. Suitablesupports include silica, alumina, Alundum, silicon carbide, titania,boron phosphate, zirconia, and the like. The catalysts may be used in afixed-bed reactor using tablets, pellets or the like, or in a fluid-bedreactor using catalysts preferably having a particle size of less thanabout 300 microns. The contact time may be as low as a fraction of asecond or as high as 50 seconds. The reaction may be conducted atatmospheric, superatmospheric or subatmospheric pressure.

EXAMPLES 1 AND 2

80 grams V₂ O₅ (99.5% powder) was introduced into 500 ml. allyl alcoholliquid medium with mechanical stirring and with reflux for about 2hours. The resulting slurry was deep green in color. The slurry wasfiltered through a fritted filter funnel to yield 73.76 grams solid upondrying. The filtration removed all unsolubilized vanadium compoundparticles having a diameter greater than about 0.04 to about 0.06 mm.7.8 grams 100% orthophosphoric acid was dissolved in 40 ml. allylalcohol and was added to the filtrate with stirring. The mixture wasrefluxed for about 16 hours. A pale powder blue precipitate (catalystprecursor) was formed which was filtered and dried for 2 hours at 150°C. The catalyst precursor was then tabletted with 1% graphite beingadded, in a Buehler press to 11/8 inch (about 2.84 cm) diameter. Thetablets were then calcined in air from 150° C. to 400° C. at a rate of5° C. per minute, being held at 400° C. for 1 hour. The resultingcatalyst had the formula V₁.0 P₁.2 O_(x).

EXAMPLES 3-5

The procedure of Examples 1-2 was followed for Examples 3 and 4, withthe exception of crotyl alcohol being utilized as the liquid mediumrather than allyl alcohol. The resulting catalyst had the formula V₁.0P₁.2 O_(x).

EXAMPLES 6-8

The procedure of examples 1-2 was followed for examples 6-8, with theexception that the liquid medium comprised the mixture of 6.35% byweight allyl alcohol (reducing agent) and 93.65% by weight tert-butylalcohol as the liquid medium. The resulting catalyst had the formulaV₁.0 P₁.2 O_(x).

The catalysts described in Examples 1-8 were used to produce maleicanhydride from butane using a 20 cc fixed-bed reactor consisting of a 38cm length of stainless steel tubing having an outer diameter of about1.3 cm and having a full length 0.31 cm axial thermowell. The reactorwas heated with a split stainless steel block furnace. Flasks forreceiving the product maleic anhydride were mounted in ice water, andtail gases were routed to a Carle Analytical Gas Chromatograph III foranalysis. Reaction conditions and results of the tests run are describedin Table I. The results are stated in terms as follows: ##EQU1##

When the process of preparing catalysts containing mixed oxides ofvanadium and phosphorus is employed according to the present invention,the hazards presented by using highly corrosive materials such as HClgas are avoided. In addition, the vanadium and phosphorus-containingcatalyst precursor can be separated from the reaction medium simply byfiltration or similar methods, avoiding the hazards of evaporating offlarge quantities of flammable liquid. The liquid reaction mediumproduced by the process of the present invention, after the catalystprecursor has been removed, may easily be recycled for use in thereaction again.

As can be seen from the results listed in Table I, catalysts preparedaccording to the process of the invention, utilizing olefinic,oxygenated organic liquids as the liquid medium or in the liquid mediumfor partially reducing vanadium, effect high yields and selectivities of4-carbon atom hydrocarbons (such as butane) or maleic anhydride.

Thus it should be apparent to those skilled in the art that the subjectinvention accomplishes the objects set forth above. It is to beunderstood that the subject invention is not to be limited by theexamples set forth herein. These have been provided merely todemonstrate operability, and the selection of vanadium andphosphorus-containing compounds, olefinic oxygenated organicliquid-containing liquid media, promoter element-containing compounds ifany, hydrocarbon feedstocks and reaction conditions can be determinedfrom the total specification disclosure provided without departing fromthe spirit of the invention herein disclosed and described, the scope ofthe invention including modifications and variations that fall withinthe scope of the attached claims.

                  TABLE I                                                         ______________________________________                                        Oxidation of n-Butane to Maleic Anhydride                                     Using V.sub.1.0 P.sub.1.20 O.sub.x Catalysts                                                 Con-                                                           Ex-  Tempera-  tact           Maleic Anhydride                                                                         Hours                                am-  ture °C.                                                                         Time    % Con- %     % Selec-                                                                             on                                 ple  Bath   Bed    (Sec.)                                                                              version                                                                              Yield tivity Stream                           ______________________________________                                        1    390    400    2     96.1   61.9  64.5   67                               2    380    389    2     89.4   61.6  68.5   72                               3    360    373    2     93     61.9  66.3   23                               4    370    384    2     95.5   63.6  66.6   42                               5    370    383    2     93.8   61.8  65.9   85                               6    435    446    1     91     54.6  60.0   20.9                             7    428    444    2     85.2   52.2  61.3   63                               8    437    457    2     91.2   54.3  59.5   93                               ______________________________________                                    

We claim:
 1. A process for the production of maleic anhydride by theoxidation of n-butane, n-butene, 1,3-butadiene or a mixture thereof withmolecular oxygen or oxygen-containing gas in the vapor phase at areaction temperature of 250° C.-600° C. in the presence of a catalystcontaining the mixed oxides of vanadium and phosphorus, wherein saidcatalyst is prepared by(a) introducing a pentavalent vanadium-containingcompound into an olefinic, oxygenated organic liquid-containing liquidmedium, (b) effecting reduction of at least a portion of said vanadiumto a valence state of about +4 and in the absence of a corrosivereducing agent; (c) adding a phosphorus-containing compound to saidmedium prior to or subsequent to effecting said reduction to form acatalyst precursor; (d) recovering the catalyst precursor; (e) dryingthe catalyst precursor; (f) calcining the catalyst precursor.
 2. Aprocess as recited in claim 1 wherein said reduction of vanadium iseffected in the presence of the phosphorus-containing compound.
 3. Aprocess for the production of maleic anhydride by the oxidation ofn-butane, n-butene, 1,3 butadiene or a mixture thereof with molecularoxygen or oxygen-containing gas in the vapor phase at a reactiontemperature of 250° C.-600° C. in the presence of a catalyst containingthe mixed oxides of vanadium and phosphorus, wherein said catalyst isprepared by(a) introducing a pentavalent vanadium-compound into anolefinic oxygenated organic liquid-containing liquid medium capable ofat least partially solubilizing the vanadium; (b) effecting reduction ofat least a portion of said vanadium to a valence state of about +4; (c)removing unsolubilized vanadium-containing compounds having a particlesize greater than about 0.1 mm diameter; (d) adding a pentavalentphosphorus-containing compound to the reaction medium resultng from step(c) to form a catatalyst precursor precipitate; (e) recovering saidcatalyst precursor precipitate; (f) drying said catalyst precursorprecipitate; and (g) calcining said precipitate.
 4. A process as recitedin claim 1 or 2 wherein said organic liquid medium is essentiallyanhydrous.
 5. A process as recited in claim 1 or 2 wherein reduction ofsaid vanadium is effected by heating the vanadium-containing liquidmedium.
 6. A process as recited in claim 3 wherein said unsolubilizedvanadium-containing compounds are removed by filtration.
 7. A process asrecited in claim 1 or 2 wherein said organic liquid is an olefinicalcohol.
 8. A process as recited in claim 7 wherein said organic liquidis selected from allyl alcohol, methallyl alcohol, crotyl alcohol andmixtures thereof.
 9. A process as recited in claim 1 or 2 wherein saidoxidation catalyst is represented by the empirical formula:

    V.sub.l P.sub.a O.sub.x

wherein a=0.5 to 2.0 and x is the number of oxygens required to satisfythe valence requirements of the other elements.
 10. A process as recitedin claim 1 or 2 wherein said oxidation catalyst is represented by theempirical formula V₁ P₁.1 o_(x).
 11. A process for the production ofmaleic anhydride by the oxidation of n-butane, n-butene, 1,3 butadieneor a mixture thereof with molecular oxygen or oxygen-containing gas inthe vapor phase at a reaction temperature of 250° C.-600° C. in thepresence of a catalyst containing the mixed oxides of vanadium andphosphorus, prepared from at least one pentavalent vanadium-containingcompound and at least one phosphorus-containing compound, wherein thepentavalent vanadium is reduced to an average valence state of about+3.9 to about +4.6, including the step of effecting reduction of atleast a portion of the dispersed or solubilized pentavalent vanadium toa valence state of +4 in an olefinic, oxygenated organicliquid-containing liquid medium and in the absence of a corrosivereducing agent.